Thin film magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the magnetic recording layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains (bits) of the magnetic grains in the magnetic recording layer. FIG. 1, obtained from Magnetic Disk Drive Technology by Kanu G. Ashar, 322 (1997), shows magnetic bits and transitions in longitudinal and perpendicular recording.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hcr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. Thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic layer and V is the volume of the magnetic grain. V depends on the magnetic layer thickness (t); as t is decreased, V decreases. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media.
For high signal to noise ratio (SNR) magnetic recording media, it is desirable to have small uniformly sized magnetic particles or grains; and a moderately low, uniform exchange coupling between the particles or grains. For longitudinal, perpendicular and tilted magnetic recording media, the optimal exchange coupling value is different (e.g., typically, a higher exchange coupling is desired for perpendicular recording media), but for each case, a constant, moderate value is desired between each neighboring grain.
Generally, low exchange coupling is desired so that magnetic switching of neighboring grains does not become too highly correlated. Reducing exchange coupling decreases the size of the magnetic particle or magnetic switching unit. Cross-track correlation length and media noise are correspondingly reduced. However, near-zero exchange coupling between magnetic grains produces a very low squareness sheared hysteresis loop, a broad switching field distribution, less resistance to self demagnetization and thermal decay, and low nucleation field (Hn) in perpendicular media designs. Non-uniform exchange coupling allows some grains to act independently while other grains act in clusters, resulting in broad distributions of magnetic particle size and anisotropy field.
Conventionally used storage media contain a magnetic recording layer having Co—Cr—Pt—B and Co—Cr—Ta alloys where B and Ta are mainly used to improve the segregation of Cr in the magnetic layer. The exchange coupling is controlled by preferentially forming non-ferromagnetic material at the boundaries between magnetic particles. Non-ferromagnetic material is commonly formed during sputter deposition of CoPtCrB containing alloys on high temperature substrates by preferential surface diffusion of Cr and B to grain boundaries. The Co concentration varies between the grain center and the grain boundary such that there is a transition from magnetic to non-magnetic composition. Exchange coupling in such media is controlled by changing parameters such as the Cr and B concentrations, and the substrate temperature.
Non-ferromagnetic material can also be formed at magnetic grain boundaries during sputter deposition of CoPt containing alloys on low temperature substrates, by addition of a metal oxide to the sputter target or by reactively sputtering the target in a sputter gas containing oxygen. Exchange coupling in these media is controlled by changing parameters such as sputter gas pressure, oxygen concentration in the sputter gas, and oxide concentration in the sputter target.
In any case, a better segregation profile of non-ferromagnetic material leads to a sharper transition between the magnetic grains and the non-magnetic Co-depleted grain boundaries. Correspondingly, the recording media can have higher Co concentration inside each grain.
However, there are several problems when sputter parameters are adjusted to control the amount of non-ferromagnetic material to create a significantly, but not completely, exchange decoupled magnetic particles. A magnetic material having a significantly, but not completely, exchange decoupled magnetic particles is termed as moderately exchange coupled.
One problem with such an exchange decoupling method is that the exchange between two grains is extremely sensitive to the arrangement of a very small number of atoms at the grain boundary between each pair of adjacent grains. Thus, some grains are much more strongly coupled than others. A second problem is that radial diffusion profiles depend upon the size of the magnetic grains. Thus, larger grains can have systematically different composition than smaller grains, and hence have systematically different exchange coupling and magnetic anisotropy. A third problem is that the composition of the entire film including the ferromagnetic particles, the weakly exchange coupled ferromagnetic regions between magnetic particles, and the exchange decoupling non-ferromagnetic regions between magnetic particles is substantially the same, except for the preferential lateral transport of some atomic species. Changing exchange coupling in the film generally changes the composition of the magnetic particles as well as the grain boundary material. Thus, it is difficult to separately optimize the properties of each component of the film. Also, a fourth problem exists related to the degraded mechanical robustness and corrosion resistance that arises from such prior art wherein high pressure sputter methods necessarily yield increased surface roughness. There is thus a need for a new method for the magnetic recording media that provides improved control of exchange coupling in the magnetic layer and improved recording performance while preserving media robustness.